In the oil and gas industries, there is a need for a threaded joint having “super high torque performance,” which numerically means an overtorque capacity of about 40% of pipe body resistance. Conventional technologies generally provide up to about 20% overtorque capacity. There is limited prior art technology which is capable of reaching super high torque performance. One technology requires a wedge thread, and is illustrated by U.S. Pat. No. Re. 34,467 (Reeves) and WO 94/29627 (Mott). U.S. Pat. No. 6,412,831 (Noel et al.) discloses a variation on the wedge thread to achieve a high tightening torque.
The tapered wedge threads described in U.S. Pat. No. Re. 34,467 are known to provide an unusually strong connection that can control the stress and strain of the connected pin and box members within acceptable levels. For this reason, wedge threads are employed in many heavy-duty applications. The wedge threads generally have a dovetail shape, with load flanks and stab flanks that diverge outwardly to create crests that are substantially wider than the contiguous roots. This creates an interlocking thread form configuration, and the threads do not rely on shoulders to bear loads caused by make up. The dovetail shape of the threads, however, presents a problem in that unless the axial alignment of the joints is perfect during make up and break down, the edges of the threads of the pin and the box may contact. Such contact can prevent the pin from completely penetrating the box during make up and can cause damage to the threads.
WO 94/29627 is another example of wedge threads, having wider crests than the respective roots and both stab flanks and load flanks generally angled in the same direction, or with positive and negative angles as conventionally measured with respect to the longitudinal axis of the connection. Such a configuration can decrease the chances that the thread edges will come into contact during make up and break down, thus, minimizing risk of thread damage.
However, the wedge thread configurations disclosed in the '627 publication, and the '467 patent have the common disadvantage of a very low energy absorption to the final torque point; a low fatigue resistance due to the high stress present in the last engaged thread; and a higher tendency for disengagement, which commonly is known as spring back. Such wedge thread connections require tight machining tolerances, which makes them difficult and expensive to manufacture, as well as difficult to inspect for defects. Still further, the acute angles of the wedge threads disposed on the load flank side and/or the stabbing flank side have sharp cuts that are deleterious to the function of the connection. Such threads also become more fragile during use as a consequence of thread root notching, that is caused by the shape angles.
U.S. Pat. No. 6,412,831 discloses a threaded connection of two metal pipes, which includes a tapered thread with one type of male trapezoidal threads on a pin element and a mating female trapezoidal thread on the box element. While the width of the thread at the crest is less than the width of the thread at the root, that is achieved by a small, negative load flank angle (a. is preferably −3°) paired with a much larger positive stabbing flank angle (b. is preferably 13°), which combine to define an included angle (δθ) between the load flank and stabbing flank surfaces that is still positive (preferably 10°). While the male and female elements are said to be able to reach a position beyond where two flanks of the male thread come into contact with two flanks of the female thread, it is emphasized that a viscous grease form of dope critically is required. (See column 8, line 46).
Hence, achieving a makeup with a higher torque requires a viscous grease with the modified wedge thread geometry illustrated in the '831 patent. This is disadvantageous. While there might not be any radial interference, the presence of the viscous dope creates a high pressure in both the crests and the roots, which diminishes the contact pressure between flanks, and results in a decrease in torque. For these reasons, as well as for environmental reasons, it is highly desirable to avoid the use of dope in order to reach a high torque value.
The advantages and disadvantages of conventional dope or grease (i.e., API 5A3, which is an API modified grease) as a thread treatment is known. A particular class of dry thread pretreatments, which do not require the application of dope or grease as a lubricant, also now are known, and such dry thread pretreatments are broadly referred to herein as “dope-free”.
Applicants incorporate by reference herein to the disclosures of U.S. Patent Application Publication No. 2005/0176592 A1 for further details of a dope-free dry lubricant coating achieved by applying a dry film including an intrinsically conductive polymer to a thread surface. Applicants also incorporate by reference herein to the disclosures of U.S. Pat. No. 6,971,681 B2, for details of dry surface treatments of threads for use in the oil and gas extraction industry that increase both the corrosion resistance and galling resistance of a connection joint.